Manufacture of thermionic cathodes for electric discharge devices



Patented Feb. 5, 1935y MANUFACTURE oF TH y FOR ELECTRIC DISCHARGE DEV1CES "John'Turton Randall,l Ruislip, England,assignor to General- Electric Co New York l n 'No Drawing. Application July 3, 1933,"seriai No.' f

inpany', a corporation ol'V 7 678,989. In Great Britain July 2z, 1932 9 claims. v(o1. 25o-27.5)

. This invention relates generally to the manufacture ofv thermionic cathodes for electric discharge devices and more particularly the invention relates tothe manufacture of cathodes of 5 the type consisting cfa self-supporting masscontaining alkaline earth oxides, generally rod-like in shape, heated by means, for example a tungsten spiral, which does 'notcontrbute materially to the vrigidity 'of the mass.v Cathodes of this type are to be distinguished from those in which the oxides form alayer on a metallic c ore which supports them and by which they .are heated; they are moet widely employed in gaseous electric discharge lamps'. f v v Y Cathodes of this type are usually ,prepared by forming aymass ofy the appropriate shape from some. compound oie-the. alkaline earth `metals which is' laterA converted into the oxide by heat. The carbonate is theLcompound most generally used. It is ya necessary consequence of this process that the resulting mass is-porous and less dense than a truly compactV mass of the oxide. This porosity is disa'dvantageous for several reasons; forexamplejit permits air to penetrate into the. interior ,ofltzhemass if' the' mass is4 exposed'to air for a shorttime't'oy disintegrate the mass by the formation of hydroxide and carbonate. The porosity can be reducedsomewhat by sintering' the mass after the carbonate has been decomposed', vbut it is not easy by thisA means to produce' a rod of. the vnecessary compactness'.

The object of the present invention is to provide a compact electrode of the above type which does not disintegrate when exposed to'air. A furtherv object of the invention is to provide an electrede of the above type which is less porous and more readily compacted by sintering than those now known in the art. VStill furtherobjects and advantages attaching to the device and toits use and operation will lbe apparent to thoseskilled in the art frornthel,followingI `particular description..

We have discovered that ,less porous masses, more readily compacted'by sintering, can be produced ifthe peroxidel is used instead of the carbonate for the starting material. We are aware that it has beenV proposed to produce thermionic cathodes by converting the peroxide of the `alkaline earth metals into oxides, but only when the catho-de has the form of a layer on a support, not when it has the form of a self-supporting mass. When the cathode is a layer on a support the qualities that we have found to result from the substitution of the peroxide for the carbonate are of little or no importance.

vy According toftheinventiona thermionic cathode of the type specified is produced by forming a mass out of materialsin which the alkaline earth metals are present asperoxide, and decomposingthe peroxide byheat y n f f u Since the main `featureof the invention is the lsubstitution of peroxide for vthe usualcarbonate, any characteristics of known icathodesiof--vthe .typelspeciiied that are` nota-necessarily aiected by the substitution can be adopted in cathodes according to the invention without exceeding its scope. Thusrnetal powder, yparticularly tungsten powder, is oftenmixed. with the alkaline earth carbonatos rand remains inthe resulting oxide. Such admixture is permissible alsoin cathodes made'in accordance with' the invention.` y

' The original mass of the ,desiredshape' is koften prepared by squirting the 'material'mixediwith'a smalll quantity, of binder. "We `have discovered that notallY binders suitable for use withcarybonate are equally suitable with peroxide; thus gum 'tragacanth'is'not a suitable binderfor peroxide. On the other hand a suitable binder isa 10% solution of nitrovoelluloseinframyl' acetate, `4 tov 5 gramsuof thisisolution being 4mixed 'with 20 grams of barium peroxide.v vSince the quality of nitro-cellulose differs markedly among different` samples, the yexact; 'composition and amount of the binder must lbe adjusted 'by trial tothe particular sample. l

The heat treatment ,of the'squirted massconsists `of ytwo' main stages. ,In the first stage the binderuand thlentheperoxide` `are decom' posedleaving the oxide. lIn thel second the oxide is sintered so as ,tow be rendered more., compact. The irstlstage-` is preferably conducted,A in a vac,- uum,A that is to sayl at aslow apressure as the evolution. 0f ses during this .Stage Willfpermit- The temperature duringthissta-ge need'notwex.-

ceed 10009 C., ythough `1150?;C..issometimesjpreferable; this vis, a temperaturemt at can ilee'fat; tained in a vacuumiurnace .of-theusualltype The ratev atv which thel t'er'npe,rature is raised @Si be. '510W lilil 4 the kinder; 1S decomposed, thereafter it may be more rapid. "The second stage is not conveniently conducted in a vacuum,

since the temperature required is too high for vacuum furnaces of the usual type, but it "may be conductedin hydrogen or other gas neutral to the oxide. l

In one example of the process according to the invention, a mixture of barium peroxide with the binder above-mentioned is squirted into rods 4.5 mm. inl diameter. The rods are placed in a vacuum furnace and the vtemperature is raised to l C. over a period of 30 minutes; it is then lraised from 125 C. to 250 C. 'over a further dicated by the fall of pressure` when the evolution of gas ceases. The rods are now removed from the vacuum furnace and placed in fa molybdenum boat llined with-tungsten'with ak furnace lled with hydrogen; the temperature is raised to 1620 C. over aperiod of 30 minutesl and heldat this temperature for about 10minutes.' They are then removed, cooled andinserted in the electric discharge device. tungsten-lined boat is necessary because barium oxide is very corrosive at these high tempera.- tures. Magnesia is a possiblersubstitute for tungsten. The temperature to which it is desirable to heat the oxide in the second stage is" limited mainly by the resistance of the boat to attack, higher temperatureswould probably be desirable if boats to stand them could be obtained. 4

` While the above described method includes Ythe step of squirting the mass into desired shape it Will'be understood, of course, that we contemplate the step of pressing the mass into desired shape, either by'compacting it in a mold or by any other method of applying pressure thereto.

When a mold is'used thev mass is left therein during the carrying out of the process, or not, as desired, the mold forming a support for the mass duringv they process and in the lamp in which the completed electrode is incorporated. .The mold,`of,course, consists of material capable of withstanding the high ytemperaturesof the process, such yas tungsten or magnesia. It will be understood, of course, -thatthe electrode leads are inserted and sint'ered in the mass during the process, when desired. VWhen the mass is y pressed into shape, as by compacting it into a mold,a binder can' frequently be dispensed with, particularly where the mold is retained thereon during the process- When no binder is used the steps in the process necessary to decompose the binder are modified accordingly. 'Ihe resulting electrode as stated has been found to be a highly eiiicient one when used as the cathode of gaseous electric discharge lampsand the like.

lWhat'we claim as new and desire to-secure by Letters Patent of the United States isz- 1.` The method of manufacturing a thermionic cathode which comprises the steps of mixing alkali earth metal peroxide with a binder consist'- ing of a solution of vnitro-cellulose in amyl acetate, shaping the mass of such material and decomposing Lthe peroxide by heat.

i 2. 'I he method of manufacturing al thermionic vcathode which comprises the steps of mixing alkali earth'metal peroxide with a binder vconsisting of a solution of nitro-cellulose in amyl acetate, shaping the massof such material and The use ofV af heating the shaped mass in a vacuum to the decomposition temperature of the peroxide.

' 3. The method of manufacturing a thermionic cathode which comprises the steps vof mixing alkali earth metal peroxide with a binder consisting ofV a solution ofnitro-cellulosein amyl acetate, Ashaping the mass of such material, heating the shaped mass in a vacuum to the decomposition temperature of the peroxide and then raising said mass to the sintering temperature thereof. l

4. The `method of manufacturing a thermionic cathode which comprisesv the `steps of vrmixing alkaliearth metal peroxide with a binder consisting of asolution of nitro-cellulose in amyl acetate, shaping the mass of suchmaterial, heat- 'ing the shaped mass in a vacuum to the decom-V position temperature of the peroxide and then raising said mass to the sintering temperature thereof in an atmosphere of neutral gas.

j V5. The method of manufacturing a thermionicV cathode which comprises the steps of mixing alkali earth metal peroxide fwitha binder consisting of a solution of nitro-cellulose in amyl acetate,

shaping the Vmass of such material, heating the shaped mass in a vacuum to the decomposition temperature ofthe peroxide, placing said mass on a tungsten support and then raising said mass to the sintering temperature thereof .inan atmosphere of neutralgas. l i

6. Ihe methodl of manufacturing a thermionic cathode which comprises'the steps Vof Vmixing alkali earth metal peroxide with a binder consisting of a solution of nitro-cellulose in amyl acetate, shaping the mass' of such material, heating the shaped mass in a vacuum to the decomposition temperature of the peroxide, placing said masson va magnesia support and then raising said mass to the sintering temperature thereof in an atmosphere ofneutral gas. k

7. The method of manufacturing a thermionic cathode which ,comprises the 'steps of forming ja mass of desired shapeout ofmaterials in which 4alkaline earth metals are present in the form of their peroxide,fdecomposing the peroxide by heat f the` peroxide, shaping the mass of such material,

heating the'` shaped mass'to the decomposition temperature first of the binder and then ofthe peroxide and then raising said mass to the sintering temperature thereof? JOHN TURTON RANDALL. 

